Excellent curable transparent rubber compositions have been described in WO2014132718. Even though in this application an improved balance has been found between rubber properties such as hardness and strength on the one hand and transparency on the other hand, there is still a need for further improvement. Moreover, this reference is silent with respect to the method for preparing a cured transparent rubber article.
JP2005002225 provides a rubber composition that comprises 100 pts. wt. rubber like polymer and 5-150 pts. wt. wet silica based on 100 pts. wt. rubber like polymer. The rubber like polymer consists of 5-80 wt. % rubber like polymer (A) component having 1.535-1.600 refractive index at 23° C., and 20-95 wt. % rubber like polymer (B) component having 1.400-1.530 refractive index at 23° C. The manner by which the composition is to be used is not disclosed.
JPH05140379 provides a composition comprising a specific maleimide copolymer and an elastomer in a specified ratio that is excellent in the heat resistance, rigidity, and mechanical characteristics, and useful for automotive lenses. The composition comprises (A) 0.5-99.5 pts. wt. of a maleimide copolymer and and (B) 99.5-0.5 pts. wt. of an elastomer such as natural rubber having a difference of <=0.01, preferably <=0.005, between the refractive index of the component A and the refractive index of the elastomer at a wavelength of 589 nm. Various methods for moulding are disclosed. The document provides no information on the use of a peroxide curing agent. Transparency and haze still leave room for improvement.
Rubber articles may be made by a variety of moulding processes. Types of moulding methods includes blow moulding, compression moulding, extrusion moulding, injection moulding, rotational moulding, thermoforming, and the like. Injection moulding for instance is a manufacturing process for producing parts by injecting material into a mould cavity, where it cools and hardens to the configuration of the cavity. For thermosetting polymer, injection moulding is not the first choice, given the risk of chemical crosslinking in the screw of the injection moulding machine.
The inventors have set out to form cured transparent rubber articles with improved mechanical properties, improved tear strength in particular. Surprisingly, it was found that a particular method of forming articles at specific conditions, outperformed alternative production methods. Attractive properties, such as transparency (i.e., total transmittance greater than 80%, with haze below 30%, both according to ASTM D1003-13), abrasion resistance (ISO 4649, method A, of less than 200 mm3), which is of interest for shoe soles and similar applications are retained.
Therefore now a new method has been found that allows the production of cured transparent rubber articles with improved mechanical properties.